Tar spot of corn, a disease caused by the fungus Phyllachora maydis, has now been confirmed in Nemaha (9/15), Doniphan (10/05), Brown (10/10), Jackson (10/11), and Atchison (10/11) counties in Kansas. Tar spot lesions are black, raised, and have a round/elliptical shape. This pathogen can survive in crop residue (Figure 1).
Figure 1. Tar spot of corn. Purple arrows are indicating a few of the tar spot lesions. Photos courtesy of Rodrigo Onofre, Plant Pathologist
Tar Spot was detected in US for the first time in 2015 and has quickly spread through the Midwest (Figure 2). To date, it has been reported in Illinois, Indiana, Wisconsin, Michigan, Minnesota, Kentucky, Iowa, Ohio, Florida, Georgia, Pennsylvania, New York, Nebraska, and Missouri. First observations in our neighboring states, Nebraska and Missouri, were made last season. This disease is favored by mild temperature (60F to 73F), high relative humidity (>75%), and a prolonged leaf wetness period (>7h). Severity of tar spot is dependent on the weather. Irrigated corn may be at particularly high risk for yield or silage loss.
Producers should consider harvesting fields confirmed to have tar spot last to mitigate additional disease spread. Because of this, scouting prior to harvest is critical.
Current management recommendations for this disease are to:
Efficacy ratings for fungicides labeled for the control of tar spot can be found at the Crop Protection Network website, link:
Figure 2. Tar Spot of Corn (Phyllachora maydis) in Kansas and surrounding states for 2022. Source: https://corn.ipmpipe.org/tarspot/
Although it is past the time period for management, now is a critical time to identify fields with tar spot as these locations may be at higher risk for the disease next year. For confirmation of tar spot, please submit samples to the K-State Plant Diagnostic Clinic and enclose a completed sample submission form. You can download the form at https://www.plantpath.k-state.edu/extension/diagnostic-lab/documents/2021_PP_DiseaseLabChecksheet.pdf.pdf
Rodrigo Onofre, Row Crop Plant Pathologist